1. Field of the Invention
The present invention relates to an optical head and an optical disk apparatus both utilizing a near-field light, particularly an optical head and an optical disk apparatus both permitting the attainment of high-density recording.
2. Description of the Prior Art
Optical disks used in optical disk drives have been becoming larger in both density and capacity from a compact disk (CD) to a digital video disk (DVD), but a still further increase of capacity is now demanded with improvement in performance of computers and in definition of display devices.
The recording density of an optical disk is basically controlled by the diameter of a light spot formed on a recording medium. Recently, as a technique for making the diameter of a light spot smaller, a technique which utilizes a near-field light in a microscope has been applied to optical recording. As conventional optical disk drives using such a near-field light there are known, for example, those disclosed in the literature Jpn. J. Appl. Phys., Vol. 35 (1996), p. 443 and U.S. Pat. No. 5,497,359.
FIGS. 23A and 23B show an optical disk drive disclosed in the literature Jpn, J. Appl. Phys., Vol. 35 (1996), p. 443. As shown in FIGS. 23A and 23B, this optical disk drive, indicated at 190, comprises a semiconductor laser 191 which emits a laser beam 191a, a coupling lens 192 which shapes the laser beam 191a emitted from the semiconductor laser 191 into a collimated beam 191b, a probe 194 which has an optical fiber 193 having been ground so as to be tapered off from an incident end 193a toward an exit end 193b and which introduces the collimated beam 191b from the coupling lens 192 through the incident end 193a, and a recording medium 195 on which data are recorded with a near-field light 191c which leaks out from the exit end 193b of the optical fiber 193.
The recording medium 195 has a recording layer 195a formed by GeSbTe as a phase change medium, which is heated by incidence thereon of the near-field light 191c, thereby inducing a phase change between crystal phase and amorphous phase. Recording is effected by utilizing a change in reflectance between both phases.
The optical fiber 193 is machined so as to have a diameter of 10 xcexcm at the incident end 193a and a diameter of 50 nm at the exit end 193b and it is coated with a metallic film 194b such as aluminum film through a clad layer 194a to prevent light from leaking out to any other portion than the exit end 193b. Since the diameter of the near-field light 191c is about the same as that of the exit end 193b, it is possible to attain a high recording density of ten GB/inch2.
In reproduction, as shown in FIG. 23B, the near-field light 191c which is low in power to an extent not inducing a phase change is radiated to the recording layer 195a and reflected light 191d therefrom is condensed to a photomultiplier tube (hereinafter referred to simply as xe2x80x9cphotomultiplierxe2x80x9d) 197 through a condenser lens 196 and is detected.
FIG. 24 illustrates an optical head of the optical disk drive disclosed in U.S. Pat. No. 5,497,359. This optical head, indicated at 50, comprises an objective lens 52 which condenses a collimated light 51 and a spherical bottom-cut SIL (Solid Immersion Lens) 54 which is disposed so that a bottom 54a thereof is perpendicular to a convergent light 53 emerging from the objective lens 52. When the collimated light 51 is converged by the objective lens 52 and a convergent light 53 thus obtained is applied to a semispherical incident surface 54b, the convergent light 53 is refracted by the incident surface 54b and is focused on the bottom 54a, whereby a light spot 55 is formed on the bottom 54a. In the interior of the SIL 54, the wavelength of light becomes shorter in inverse proportion to the refractive index of the SIL 54, so that the light spot 55 also becomes smaller proportionally. Most of the light focused to the light spot 55 is totally reflected toward the incident surface 54b, but a portion thereof leaks out as a near-field light 57 from the light spot 55 to the exterior of the SIL 54. If a recording medium 56 having a refractive index almost equal to that of the SIL 54 is disposed at a distance sufficiently smaller than the wavelength of light from the bottom 54a, the near-field light 57 will be coupled with the recording medium 56 into a propagation light which is propagated within the recording medium 56. With this propagation light, information is recorded in the recording medium 56.
If the SIL 54 is constructed so that the collimated light 51 is converged at a position spaced r/n (r stands for the radius of the SIL) from a center 54c of the semispherical surface 54b, which structure is designated a Super SIL structure, it is possible to diminish a spherical aberration caused by the SIL 54 and increase the numerical aperture in the interior of the SIL 54. Further, it becomes possible to make the light spot 55 very small. More particularly, the light spot 55 is microminiaturized like the following expression:
Dxc2xd=kxcex/(nxc2x7NAi)=kxcex/(n2xc2x7NAo)
where,
k: a proportional constant (usually 0.5 or so) which is dependent on the intensity distribution of light beam
xcex: wavelength of light beam
n: refractive index of SIL 54
NAi: numerical aperture in the interior of SIL 54
NAo: numerical aperture of incident light on SIL 54
Since the collimated light 51 is converged as the light spot 55 without being absorbed on the optical path, there is attained a high light utilization efficiency. Consequently, it is possible to use a light source of a relatively low output and the detection of reflected light can be done even without using the photomultiplier.
According to the above conventional optical disk drive 190 it is possible to form a small light spot of several ten nm or so on the recording medium, but since the optical fiber 193 is tapered, a portion of laser light incident on the optical fiber 193 is absorbed in the interior of the optical fiber, thus giving rise to the problem that the light utilization efficiency becomes as low as 1/1000 or less. Therefore, the use of the photomultiplier 197 for the detection of reflected light 191d is unavoidable, with consequent increase in size and cost of the optical head portion. In addition, the response speed of the photomultiplier 197 is low and the optical head portion is heavy, so it is impossible to effect a high-speed tracking and hence impossible to rotate the optical disk at high speed. As a result, there occur various problems, including the problem of a low transfer rate, and thus improvements are needed for practical application of such a conventional optical disk drive.
FIG. 25 is a diagram for explaining a problem of the conventional optical head 50 shown in FIG. 24, which diagram is of an analysis made by Suzuki at #OC-1 of Asia-Pacific Data Storage Conference (Taiwan, ""97. 7.). A relation between the refractive index, n, of the SIL 54 and NAo is shown therein. There is a reciprocal relation between NA of incident light on the SIL 54, i.e., maximum value, xcex8max, of incident angle xcex8, and the refractive index, n, of the SIL 54. It is not that both can be made large each independently. As is seen from the same figure, with an increase in refractive index, n, of the SIL 54, the maximum value NAomax which the NAo of incident light can take becomes smaller. This is because if NAo increases beyond the maximum value Naomax and the incidence angle becomes larger as a result, then the light concerned enters the recording medium 56 directly without passing through the SIL 54 and consequently the light spot 55 rather expands at the position of the recording medium 56. For example, when the refractive index, n, is equal to 2, the value of NAomax is 0.44 and the product of the two, nxc2x7NAomax, falls under the range of 0.8 to 0.9 no matter how the two may be combined. This is a theoretical limit and an actual value is still smaller (0.7 to 0.8).
As to a light converging experiment using the Super SIL structure in question, B. D. Terris, et al. present a report in Appl. Phys. Lett., Vol. 68, (""96), p. 141. According to this report, when a Super SIL having a refractive index, n, of 1.83 is disposed between an objective lens and a recording medium and a laser beam with a wavelength of 0.83 xcexcm is converged thereby, there is obtained a light spot diameter of 0.317 xcexcm. That is, a light convergence corresponding to Dxc2xd=xcex/2.3 is attained. But in this case, NA is 0.4 and nxc2x7NAmax is 0.73 or so. Moreover, using this system, the possibility of a recording density of 0.38xc3x97Gbits/cm2, which is about several times as large as that obtained in the prior art, is verified therein.
Thus, according to the conventional optical head 50, the light utilization efficiency is high, but since there is a reciprocal relation between the refractive index, n, of SIL and the maximum value NAomax, the product of the two, nxc2x7NAomax, encounters a theoretical limit of 0.8 to 0.9, actually 0.7 to 0.8. Consequently, even with use of a laser beam having a wavelength of 400 nm, the light spot obtained will be 0.2 xcexcm or so at most in diameter. Thus, in comparison with the conventional optical disk drive which uses the probe 194 for converging light, the light spot diameter is several times or more larger and hence it is impossible to attain a high recording density.
Accordingly, the present invention provides an optical head and an optical disk apparatus which are small-sized and which permit the attainment of a high recording density.
According to the present invention, there is provided an optical head which converges a laser beam into a light spot. The optical head has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface onto which is adhered a hologram having a function of converging the laser beam and on which the laser beam emitted from the laser beam emitting unit is incident through the hologram and a second surface on which the laser beam incident on the first surface is focused to form the light spot, and a light shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction it is possible to converge the laser beam even without using an objective lens because there is used a hologram having a laser beam converging function. In addition, the use of such a hologram permits the attainment of a high NA value, whereby a very small light spot can be formed on the second surface of the transparent light converging medium. Since the light spot formed on the second surface is shaded by the shading film having an aperture whose width is smaller than the diameter of the light spot formed on the second surface, there is obtained a very small near-field light spot which is smaller than the light spot formed on the second surface.
According to another aspect of the present invention, the optical head has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface on which the laser beam emitted from the laser beam emitting unit is incident and which has a function of diffusing the incident laser beam, a second surface which reflects the laser beam incident on the first surface, and a third surface onto which is adhered a hologram having a function of reflecting and converging the laser beam and which causes the laser beam reflected by the second surface to be reflected and converged into a light spot on a different surface by the hologram, and a shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction, the laser beam can be converged even without using an objective lens because there is used a hologram having a laser beam reflecting and converging function. With such a hologram, it becomes possible to obtain a high NA value and form a very small light spot on the transparent light converging medium. Since the light spot formed on the surface of the transparent light converging medium is shaded by the shading film having an aperture whose width is smaller than the light spot diameter, there is obtained a very small near-field light spot on the transparent light converging medium.
According to another aspect of the present invention, there is provided an optical disk apparatus having an optical head which converges a laser beam to form a light spot on a rotary disk and which records or reproduces information with use of the light spot. The optical head has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface onto which is adhered a hologram having a function of converging the laser beam and on which the laser beam emitted from the laser beam emitting unit is incident through the hologram and a second surface onto which the laser beam incident on the first surface is converged to form the light spot, and a shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction, the use of a hologram having a laser beam converging function permits the laser beam to be converged even without using an objective lens. As a result, the size in the vertical direction becomes smaller and therefore it is possible to reduce the size of the optical disk apparatus. In addition, since the hologram having a laser beam converging function is used, it becomes possible to obtain a high NA value and hence possible to form a very small light spot on the second surface of the transparent light converging medium. Further, since the light spot formed on the second surface is shaded by the shading film having an aperture whose width is smaller than the diameter of the light spot formed on the second surface, there is obtained a very small near-field light spot smaller than the light spot formed on the second surface.
According to another aspect of the present invention, there is provided an optical disk apparatus having an optical head which converges a laser beam to form a light spot on a rotary disk and which records or reproduces information with use of the light spot. The optical head has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface on which the laser beam emitted from the laser beam emitting unit is incident and which has a function of diffusing the laser beam, a second surface which reflects the laser beam incident on the first surface, and a third surface onto which is adhered a hologram having a function of reflecting and converging the laser beam and which causes the laser beam reflected by the second surface to be reflected and converged into a light spot on a different surface by the hologram, and a shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction, since there is used a hologram having a laser beam reflecting and converging function, the laser beam can be converged even without using an objective lens. As a result, the size in the vertical direction becomes smaller and hence it is possible to attain the reduction in size of the optical disk apparatus. In addition, the use of such a hologram permits the attainment of a high NA value and the formation of a very small light spot on the transparent light converging medium. Further, since the light spot formed on the surface of the transparent light converging medium is shaded by the shading film having the aforesaid aperture, there is obtained a near-field light spot smaller than the light spot formed on the transparent light converging medium.
According to another aspect of the present invention, there is provided an optical disk apparatus having plural rotary optical disks arranged coaxially at a predetermined spacing and also having plural optical heads which converge laser beams to form light spots on the plural optical disks respectively and which record or reproduce information with use of the light spots. Each of the optical heads has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface onto which is adhered a hologram having a function of converging the laser beam and on which the laser beam emitted from the laser beam emitting unit is incident through the hologram and a second surface onto which the laser beam incident on the first surface is converged to form the light spot, and a shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction, the laser beam can be converged even without using an objective lens because a hologram having a laser beam converging function is used. As a result, the size in the vertical direction becomes smaller and it is possible to diminish the spacing between adjacent optical disks. Moreover, the use of the hologram having a laser beam converging function permits the attainment of a high NA value and the formation of a very small light spot on the second surface of the transparent light converging medium. Further, since the light spot formed on the second surface is shaded by the shading film having an aperture whose width is smaller than the diameter of the light spot formed on the second surface, there is obtained a very small near-field light spot smaller than the light spot formed on the second surface.
According to another aspect of the present invention, there is provided an optical disk apparatus having plural rotary optical disks arranged coaxially at a predetermined spacing and also having plural optical heads which converge laser beams to form light spots on the plural optical disks, respectively, and which record or reproduce information with use of the light spots. The optical head has a laser beam emitting unit that emits the laser beam, a transparent light converging medium having a first surface on which the laser beam emitted from the laser beam emitting unit is incident and which has a function of diffusing the laser beam, a second surface which reflects the laser beam incident on the first surface, and a third surface onto which is adhered a hologram having a function of reflecting and converging the laser beam and which causes the laser beam reflected by the second surface to be reflected and converged into a light spot on a different surface by the hologram, and a shading film formed on the second surface of the transparent light converging medium and having an aperture in a position where the shading film shades the light spot, the aperture having a width smaller than the diameter of the light spot.
According to this construction, with use of a hologram having a laser beam reflecting and condensing function, the laser beam can be converged even without using an objective lens. As a result, the size in the vertical direction becomes smaller and it is possible to diminish the spacing between adjacent optical disks. Further, the use of such a hologram makes it possible to obtain a high NA value and form a very small light spot on the transparent light converging medium. Additionally, since the light spot formed on the transparent light converging medium is shaded by the shading film having an aperture whose width is smaller than the diameter of the light spot, there is obtained a very small near-field light spot smaller than the light spot formed on the transparent light converging medium.